The present invention relates to thermal insulating material for use in construction of homes, factories and office buildings. More particularly, the present invention relates to a thermal barrier comprising layers of thin walled hollow tubes stacked together so that their longitudinal axes are aligned in a plane perpendicular to the direction of heat flow.
Efficient utilization of thermal insulation continues to be one of the best methods of conserving energy and controlling heating and cooling costs. At the same time, the health and safety concerns associated with conventional insulating materials, such as mineral fibers and plastic foams, has raised serious questions about their long term applicability. E.g., recent discoveries point to the possibility that fiberglass of certain particle sizes may be carcinogenic in the same or similar manner as asbestos. In addition, the performance of mineral fibers is relatively limited, with typical R-values being about 2.5 per inch. The use of plastic foams, such as polyurethane, urea-formaldehyde and polystyrene, has also been criticized because of their dimensional instability and tendency to evolve their blowing gases and/or unreacted constituents, which significantly lowers their performance. Moreover, they are flammable and many generate poisonous or noxous gases as they burn.
Efforts are underway to improve this situation by developing new kinds of materials. One of these newly developed insulation materials utilizes packed glass microspheres coated with a reflective material and having a vacuum in the intersticial area between microspheres. The outer reflective coating is intended to minimize heat transfer by radiation; the vacuum in the intersticial area reduces heat transfer by gas conduction. Although insulation materials made from these types of microspheres possess distinct advantages over commercially available materials, they also have several inherent disadvantages. For example, it has been found difficult, if not impossible, in many applications to maintain the vacuum in the intersticial area. This dramatically increases energy losses induced by gas conduction. It has also been found to be very difficult to deposit a relatively thin film of reflective material on the outer surface of the microspheres. Even where this has been accomplished, the coating wears at the area of point to point contact between microspheres. The point to point contact, in and of itself, increases heat transfer by solid conduction and the wearing of the reflective material necessarily increases heat transfer by radiation. Moreover, the known methods of producing hollow glass microspheres, e.g., U.S. Pat. Nos. 2,797,201 and 3,365,315, etc. have not been successful in producing products of relatively uniform size or uniform thin walls. This makes it difficult to produce insulation materials of controlled and predictable characteristics and quality.
Another packed glass microsphere insulating material is taught in U.S. Pat. No. 4,303,061. In that invention, thin walled hollow microspheres contain a vacuum and a reflective coating on the interior surface. The microsphere is formed from molten glass and the vacuum and reflective coating are put in place at the time the microsphere is made. According to the patent, all of these operations are conducted at the melting temperature of glass, about 2,000.degree. F., an extremely high and difficult environment in which to carry out such procedures. To further compound this difficulty, glass evolves water at these temperatures. The water, in turn, can be reduced by the metals comprising the reflective coating to form hydrogen. Since hydrogen is an excellent conductor of heat this result can have a deleterious effect on the insulation property of the microspheres. Equally as important, at these temperatures it is extremely difficult to evenly deposit a reflective coating of the desired thickness on the interior surface of the microsphere.
A need thus exists in the art for a thermal barrier which is not subject to the deficiencies of either the present commercially available materials or the packed glass microsphere technology, which can be readily manufactured with predictable characteristics and quality, and which can be mass produced at prices attractive to the construction industry.